24 ESSAYS ON BACTERIOLOGY. 



more striking than tlie contrast betwpfen disinfection 

 -before and since tbe recent growth of bacteriology. 

 Heretofore it has been in some respects almost like 

 beating the air Or fighting in the dark. Now the gen- 

 eral problem of disinfection has reduced itself to this: 

 How most efficiently and with least danger to insure 

 the destruction of disease germs. The second condi- 

 tion is important because many germicidal agents are 

 active poisons. 



Definite knowledge having been reached as to just 

 what is to be destroyed, experimenters are at once 

 able to drive at the heart of the first problem of disin- 

 fection: What agents will destroy these bacteria, 

 and what is their relative germicidal power? As the 

 result of these studies, bi-chloride of mercury stands 

 at the head of chemical agents, heat of the non-chem- 

 ical; and perhaps the most important result of the in- 

 vestigation of disinfectants is the high place given to 

 heat, not only in theory but in practice. 



Another interesting and important set of facts has 

 been brought out, as the result of which disinfectants 

 have been classified, as to their activity, as follows: 

 First, those which rapidly destroy both bacteria and 

 spores; second, those which destroy mature bacteria 

 but not spores; third, those which prevent the devel- 

 opment of bacteria or spores, and, finally, those which 

 more or less retard their development. It will at once 

 be seen that this classification is of more than theoret- 

 ical interest. For, as is well known, many if not all of 



